Vacuum coating techniques are known and used in the production of coated substrates for numerous applications including, for example, thin film optical devices, such as optical substrates having reflective, anti-reflective, wavelength selective or other thin film coatings to perform as filters, mirrors, interleavers, wide band pass filters, narrow band pass filters, edge filters, short wave pass filters, long wave pass filters and other devices useful in optical communication systems, sensors and other applications.
Physical vapor deposition (PVD) of thin films covers a broad class of vacuum coating deposition processes in which material is removed from a target or source and deposited onto the surface of a substrate, the process being carried out in a vacuum or partial vacuum (both being referred to herein simply as a vacuum). Exemplary physical vapor deposition techniques include sputter deposition in which material is physically removed from a source or target by sputtering, transfers or travels across a vacuum zone within the vacuum chamber, and condenses or deposits as a coating or film on the surface of the substrate. In reactive sputtering the coating material generated at the target typically reacts with an introduced gas, such as oxygen as or after it deposits on the substrate surface as a coating of the reaction product. Variations of these and other vapor deposition techniques are known, including, ion assisted, e.g., diode or triode sputtering, ion beam sputtering, ion assisted evaporation, planar or cylindrical magnetron sputtering, direct current or radio frequency sputtering, electron beam evaporation, activated reactive evaporation and arc evaporation.
For many applications, the thin film coating on a substrate must be deposited with extreme precision. Optical elements for sensors and optical communication devices, such as wavelength selective thin film filters, mirrors, interleavers and the like useful in optical communication systems, are commercially produced by coating wafers using magnetron sputtering, ion beam sputtering or other vacuum coating deposition techniques. The coated wafer is cut into numerous small optical elements. Typically such optical elements comprise multiple Fabry-Perot film stacks, i.e., multiple resonant cavities each formed of numerous alternating thin films of high and low refractive index materials deposited in sequence onto the surface of an optically transparent wafer. Frequently such optical elements employ multi-cavity coating designs, as in a Fabry-Perot bandpass filter selectively transparent to the narrow wavelength band at the assigned center wavelength of a particular channel in a multi-channel system employing wavelength division multiplexing. To form such optical elements, each cavity (itself a filter) forming the multi-cavity Fabry-Perot coating must be resonant at precisely the same wavelength. That is, the cavities must be optically matched to each other. It is a well-known problem in this industry, however, that coating quality is difficult to adequately control. Typically, monitoring systems used during the deposition monitor the growing film thickness at the monitored point on the wafer. The uniformity and stability of the deposition substantially impacts the yield obtained from the rest of the wafer. The precise thickness and uniformity of each of the multiple layers deposited on a substrate wafer controls the optical properties of the resulting optical elements. Unfortunately, however, considerable uncontrolled variation occurs from one location to the other on the wafer surface during deposition. Variation occurs in film deposition rate from point to point on the surface of the substrate during the deposition, which can result in film thickness differences at different locations on the wafer. Also, considerable uncontrolled variation occurs in film deposition rate over time during a deposition run, potentially resulting in the different stacked cavities forming the filter coating being mismatched. Consequently, much of the surface area of a coated wafer produced in a typical deposition run either has the wrong passband or other optical property or is effectively opaque due to having optically mismatched Fabry-Perot cavities. Thus, yields of useable optical elements from a wafer coated in a typical deposition run are lower than desirable.
As a result of these difficulties, it is a significant problem that the cost of producing such optical elements is high. The problem is exacerbated by the growing need for increased quantities of such optical elements. Their use in sensors, medical devices and other applications is advancing and, most notably, the quantities used in optical communication systems are rapidly increasing in the face of shortages. So-called bandwidth-hungry applications such as the Internet, e-commerce, video-on-demand, public and private web sites and the like are driving rapid installation and upgrading of optical communication system capacity. Moreover, the problem is yet further exacerbated by the demand for ever narrower bandwidth optical elements to accommodate narrower channel spacing in optical communication systems to increase effective bandwidth by dividing the usable spectrum into an ever larger number of multiplexed channels. Equipment and processes currently used in the production of bandpass filters and other Fabry-Perot optical elements have exceedingly low yields of such higher quality optical elements, resulting in costs which are detrimental to implementation of higher density wavelength division multiplexing (WDM) in fiber optic telecommunication systems. As the wavelength spacing between adjacent WDM channels decreases, the precision and quality of the optical elements for thin film filters, mirrors, interleavers and the like must increase. This can be expected to reduce yields, as discussed above, yet yields of optical elements from each deposition run must be increased rather than decreased in order to reduce the cost of the optical elements.
Various suggestions have been made for improving and controlling film deposition.
A modified reactive magnetron sputtering apparatus is suggested in U.S. Pat. No. 4,931,158 to Bunshah et al, wherein a wire grid is positioned over the target and an auxiliary plasma is said to be produced adjacent to the substrate using a positively biased d.c. probe. Deposited film properties are said to be controlled by varying the d.c. probe voltage and the open area of the wire grid.
It is an object of the present invention to provide improved methods and apparatus for PVD vacuum coating deposition, especially sputter deposition. It is a particular object of certain preferred embodiments of the invention to provide methods and apparatus for vacuum coating deposition, which are suitable for producing high precision optical elements for fiber optic communication systems. Other objects and aspects of the invention, generally, or of certain preferred embodiments, will be apparent from the following summary disclosure and/or from the detailed description thereafter of certain preferred embodiments.
In accordance with various aspects of the inventive technology disclosed here, film deposition is controlled to produce coated substrates having large areas of operative optical coating, for example wafers having large areas suitable to be cut into optical elements operative as high quality Fabry-Perot filters. Without being bound by theory, it may be useful to an understanding of this disclosure to consider that the coating material transferring to the substrate from the target in a sputtering or other PVD process, by application of the various aspects of this inventive technology, is better controlled to reduce variation in the individual films making up the finished Fabry-Perot coating on the substrate. As will be better understood from the following disclosure and the detailed discussion of certain preferred embodiments, the deposition apparatus and systems and the deposition methods disclosed here in some aspects minimize or reduce variation, and in some aspects control variation in the coating material to either reduce or at least temporarily create deposition variation across the substrate to achieve net uniformity or near uniformity or controlled non-uniformity of the deposited films.
In accordance with a first aspect, deposition apparatus and systems are provided for vacuum coating deposition of thin film coatings onto the surface of substrates, such as optical glass wafers, etc. The vacuum coating deposition apparatus comprises a vacuum coating deposition chamber and a superstructure that provides mounting for vacuum coating deposition components within the chamber. The superstructure provides mounting sites for deposition components, such that these components are in fixed position relative each other during the deposition process. Preferably the coating deposition components are mounted to the superstructure in adjustably fixed position, as further disclosed below. The superstructure is structurally isolated from the chamber, meaning that it is at least substantially isolated from such vibrations and flexure as are typically experienced by a deposition chamber during a deposition process. A vacuum deposition chamber, meaning generally the shell, vacuum pumps and other integrated and permanently or semi-permanently attached components which form the vacuum chamber, may experience environmental vibrations and resonant waves from pumping components and the like. Flexure may occur in the chamber walls as a result of temperature changes during the deposition process and due to the pressure differential versus atmosphere as a vacuum is pulled in the chamber. Reference here in the singular to the xe2x80x9cwallxe2x80x9d of a vacuum chamber should be understood to mean generally the entire air-tight shell, (including, e.g., its door) regardless of the construction or shape of the chamber unless otherwise clear from context.
In accordance with certain preferred embodiments, the superstructure is contained within the deposition chamber. It may comprise a rigid, freestanding member or assembly standing on the floor of the chamber or otherwise mounted therein in any manner effective to substantially isolate it from the aforesaid vibrations and flexure of the chamber. Exemplary arrangements include freestanding the superstructure on dampening pads on the chamber floor or the like, such that it is independent of sidewall flexure and chamber vibrations. The superstructure itself is sufficiently rigid that the deposition components mounted to it are kept in stable position relative one another during the deposition process notwithstanding any such flexure and vibrations.
In accordance with other preferred embodiments, the rigid superstructure is externally anchored or supported, i.e., it is supported outside the deposition vacuum chamber. A portion of the superstructure, referred to here as the chamber portion, may pass through the chamber walls into the chamber, with suitable vacuum seals between them. The chamber preferably is independently supported, either directly on a facility floor or on a support structure. Alternatively, in certain embodiments, it may be supported by the superstructure, e.g., in essentially hanging fashion on a structural member of the superstructure. The chamber portion, i.e., one or more portions or members of the superstructure that provide or serve as mounting sites for the vacuum coating deposition components in the chamber, may extends through an opening in the wall of the deposition chamber or present a mounting site flush with or otherwise proximate such chamber wall openings. Vacuum coating deposition components, e.g., a rotary drive mount for a substrate fixture, a target or source mount, optionally coating monitors, etc., are disposed within the vacuum coating deposition chamber. They are, however, mounted to the chamber portion of the superstructure. The vacuum seals mentioned above, between the chamber wall and the superstructure, advantageously are movement-isolating vacuum seals, such as bellows seals or other suitable seals. The mounting member or chamber portion of the superstructure establishes a fixed relative position or xe2x80x9cpositionally rigid interconnectionxe2x80x9d between the deposition components inside the deposition chamber. The mounting member or chamber portion of the superstructure may in certain embodiments be viewed more readily as part of the mounted component and in other embodiments may be viewed more readily as part of the main construct of the superstructure, i.e., as an extended part of the superstructure. It will be apparent to those skilled in this area of technology, that numerous alternative configurations and approaches are suitable for mounting the components to the superstructure. Accordingly, for ease of reference and consistency, such member or portion of the rigid superstructure, i.e., the mounting bracketry and other mounting devices etc., including any portion that passes through the deposition chamber wall (or, if external, sits at an opening in the chamber wall) may be referred to in this discussion and in the appended claims as part of the superstructure. In any event, however, it will be recognized by those skilled in the art, given the benefit of this disclosure, that such mounting member(s) or structure is part of the rigid mounting of the deposition components to the superstructure.
The term xe2x80x9cvacuum coating deposition components,xe2x80x9d as that term is used here and in the appended claims, means at least certain components employed during operation of the apparatus to coat a substrate, typically including, for example, a substrate mount, e.g., a rotary drive mount,coating thickness monitors or sensors, target or other coating material source device. In any event, the vacuum coating deposition components include at least those components whose relative position, i.e., distance from and orientation to each other, inside the chamber effects the uniformity of the coating being formed on the substrate, such as the uniformity of its thickness across the substrate surface. As discussed below, reference here to the relative position of these components being fixed is not intended to mean that their position relative to one another cannot be adjusted. Rather, it means at least that their relative position remains substantially unchanged unless and until adjusted and also remains substantially unaffected by vibrations and flexure in the chamber.
In accordance with certain preferred embodiments, the rigid superstructure of the vacuum deposition apparatus disclosed here further comprises an anchor portion. The anchor portion may comprise a single point or multipoint footing adapted to be seated on or received in a supporting surface. In accordance with certain preferred embodiments, the superstructure is anchored in the ground, extending in self-supporting fashion upwardly from at least one end which is sufficiently deep (pier-like) into the ground to be substantially isolated from vibrations of the building in which the deposition vacuum chamber is housed and operated. The vacuum coating deposition apparatus comprising a rigid superstructure can be used to excellent advantage to deposit high precision thin film coatings with improved uniformity across large substrate area(s).
In accordance with method and product aspects of the technology disclosed here, coated substrates, for example, optical glass wafers, are coated in the vacuum deposition apparatus disclosed above comprising a rigid superstructure. Typically, the substrate is coated with a Fabry-Perot thin film coating so as to be suitable to be divided into multiple band pass filters or other optical elements. Such products can be coated in a deposition chamber employed with the superstructure feature disclosed here, in accordance with known deposition materials and techniques. In accordance with preferred embodiments, however, additional aspects of the technology now disclosed are advantageously employed to provide synergistic improvement in deposition quality and uniformity to achieve excellent overall process control and yield improvement.
In accordance with another aspect, PVD coating precision and quality are improved by deposition control methods and apparatus disclosed here. Specifically, vacuum coating deposition methods and apparatus in accordance with this aspect of the disclosed technology employ a vacuum chamber having a chamber wall. A substrate mount is provided to hold a substrate, e.g., an optical glass wafer to be coated to form high precision optical filter elements, with the substrate working surface, i.e., the substrate surface to be coated, exposed in the chamber. A coating material source apparatus also is provided, comprising a target mount to present the working surface of a target or source material in the chamber. In certain preferred embodiments, the coating material source apparatus is an ion beam sputtering apparatus having a target and an ion gun directing ions toward the target to etch target material during the sputtering process. Exemplary ion beam sputtering apparatus operative in the methods and apparatus disclosed here are known to those skilled in the art, including the ion beam apparatus shown in U.S. Pat. No. 4,793,908 to Scott et al, the entire disclosure of which is hereby incorporated by reference for all purposes. In alternative embodiments, for example, the coating material source apparatus is a magnetron sputtering cathode having a target mount adapted to hold a sputtering target to present the target working surface generally toward the substrate, and having a magnet assembly positioned behind the target mount to exert a confining force on the coating material sputtered from the target working surface during PVD coating of the working surface of a substrate exposed in the chamber. The deposition apparatus further includes componentry for continual monitoring of at least one parameter of the coating deposition and for generating a corresponding control signal. In accordance with certain preferred embodiments, such componentry monitors at least film thickness. More particularly, the deposition apparatus disclosed here, in accordance with such preferred embodiments, is operative to continually monitor film thickness for thin film deposition with excellent film thickness uniformity in each film of an H/L/H type film stack of alternating high and low refractive index materials on an optical substrate. Deposition process parameters continually monitored in various alternative embodiments include, for example, coating apparatus conditions, process conditions, product conditions (i.e., the film or film stack and/or the substrate being coated) and the like. Exemplary parameters suitable for continual monitoring as disclosed here include time elapsed during deposition, thickness and/or optical properties of the individual film being deposited or the multi-layer coating being constructed, time elapsed using the current sputter target, power used in deposition of the current film or with the current sputter target, film thickness, e.g., film thickness at the center of a wafer being coated, at a radius r greater than zero from the center, or at two or more places on the substrate, the distance between the target working surface and the substrate working surface (i.e., the surface being coated), the distance between the target working surface and the magnets in a magnetron sputtering cathode, the temperature of the magnets, the target and/or the substrate, the sputtering voltage, etc. In this regard, it will be understood that monitoring may comprise measuring a value of the parameter in question, detecting the existence of a condition, accumulating and integrating numeric or other data, etc. Monitoring devices, meters and sensors suitable for monitoring various parameter of the coating deposition and for generating a corresponding control signal are commercially available and their use in deposition apparatus and methods in accordance with the principles disclosed here will be readily understood by those skilled in this area of technology given the benefit of this disclosure. The monitoring of a process parameter may involve measuring a value of the parameter and generating a signal corresponding to the measured value. The distance between the target working surface and the substrate surface can be continually measured in accordance with certain especially advantageous embodiments of this aspect of the technology disclosed here. Optical position monitors can be used through view ports in the chamber wall, optionally using known reference points and a triangulation technique or other suitable technique to determine the distance. The control signal generated in this case preferably corresponds to the magnitude of difference between the measured distance and a preselected distance. Monitoring a process parameter, as noted above, may involve detecting the existence or non-existence at the given point in time of a certain condition, such as the attainment of a half-wave film thickness condition or odd QWOT film thickness or other desired film thickness or condition. The signal generated by the sensor or meter may in this case be binary, corresponding to the yes/no nature of the process parameter. In certain embodiments the control signal value may be obtained from a pre-stored look-up table based on the measured value or detected condition. Such look-up table may be stored on any suitable memory device, such as resident or remote ROM memory or the like available to the monitoring component.
The deposition apparatus in accordance with certain embodiments includes componentry for continual control of at least one process variable of the coating deposition operation in the deposition chamber. In accordance with preferred embodiments, such control componentry is responsive to the aforesaid control signal for automatic control of the process variable and optionally is adjusted at a preset rate. The control signal also may be displayed on any suitable user interface, preferably a graphical user interface (GUI) such as an oscilloscope, or an alarm, etc. for responsive action as appropriate by an operator. As used here, process variable control may be carried out to hold the process variable constant during all or some portion of the deposition. Alternatively, the continual control may involve changing the process variable, especially where adjustment of the process variable is used to achieve more uniform or otherwise higher quality or higher yield products. Exemplary of the latter, the relative position (i.e., distance and/or orientation) of the target working surface and the substrate may be continually controlled in response to measured film thickness at multiple locations on a wafer as a Fabry-Perot coating is being constructed in the deposition chamber. Other exemplary process variables suitable for continual control as disclosed here include, but are not necessarily limited to, gas pressure in the vacuum chamber, the position of the target relative to the magnets of a magnetron cathode assembly, power level, temperature of the target, temperature of the substrate, temperature of the magnets etc. In this regard, it will be recognized that in some cases suitable control componentry is commercially available, and in other cases its construction and use in accordance with the principles disclosed here will be readily apparent to those skilled in this area of technology given the benefit of this disclosure. Optionally, the deposition apparatus comprises a rotary drive operatively connected to the sputtering target to impart rotation to the sputtering target during a coating deposition operation.
It should be understood that continual monitoring and continual control as disclosed here may in certain embodiments be implemented as continuous monitoring and control. Alternatively, such continual monitoring and control may involve regularly recurring or frequent monitoring and control suitable to the deposition parameter and the process variable in question and to the desired result. Continuous or more frequent adjustment of the position of the target, for example, may in certain embodiments provide enhanced results over less frequent adjustments. Recurring monitoring and control may in certain embodiments be carried out at regular time intervals, on a preselected schedule or upon the occurrence of preselected events or conditions. It will be within the ability of those skilled in this area of technology, given the advantage of this disclosure, to readily determine suitable monitoring and control frequency to accomplish an intended level of process control.
In accordance with especially preferred embodiments, the continual monitoring of one or more deposition parameters and the continual control of at least one process variable of the coating deposition establish a closed-loop feedback control system for the deposition. Improved deposition uniformity is achieved. Preferred and exemplary process parameters to be monitored are further discussed below along with preferred and exemplary process variables to be controlled based on the control signal generated corresponding to the measured value of the parameter or detected condition, etc. Moreover, other suitable process parameters to be monitored and process variables to be controlled will be readily apparent to those skilled in this area of technology given the benefit of this disclosure.
In accordance with especially preferred embodiments, vacuum coating deposition apparatus is disclosed here, comprising:
a vacuum-tight sputtering deposition chamber having a chamber wall;
an optical glass wafer substrate mounted in a substrate mount and having a substrate working surface exposed in the chamber for coating;
a sputtering target mount;
a sputtering target mounted by the target mount having a target working surface in the chamber;
an optical monitor for continual monitoring of actual or optical film thickness at two locations on the substrate working surface during deposition of a film onto the substrate working surface and for generating corresponding control signals, the target mount being operative to adjust the position of the target working surface relative to the substrate working surface in closed-loop response to the control signals received from the optical monitor corresponding to film thickness. Preferably, the sputtering apparatus is ion beam sputtering apparatus.
From the forgoing disclosure, it will be readily understood by those skilled in the art, that the methods and apparatus of the present invention represent a significant advance in the technology of vacuum coating deposition. In accordance with certain preferred embodiments, improved coating precision and increased yields can be achieved. Additional advantages and other aspects of the invention will be understood from the following more detailed description of certain preferred embodiments, in conjunction with the drawings.